linux-kernel - Re: [PATCH v2 1/1] KVM: arm64: allow the VM to select DEVICE_* and NORMAL

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Message-ID: <20231206164802.GT2692119@nvidia.com>
Date:   Wed, 6 Dec 2023 12:48:02 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Marc Zyngier <maz@...nel.org>, ankita@...dia.com,
        Shameerali Kolothum Thodi 
        <shameerali.kolothum.thodi@...wei.com>, oliver.upton@...ux.dev,
        suzuki.poulose@....com, yuzenghui@...wei.com, will@...nel.org,
        ardb@...nel.org, akpm@...ux-foundation.org, gshan@...hat.com,
        aniketa@...dia.com, cjia@...dia.com, kwankhede@...dia.com,
        targupta@...dia.com, vsethi@...dia.com, acurrid@...dia.com,
        apopple@...dia.com, jhubbard@...dia.com, danw@...dia.com,
        mochs@...dia.com, kvmarm@...ts.linux.dev, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 1/1] KVM: arm64: allow the VM to select DEVICE_* and
 NORMAL_NC for IO memory

On Wed, Dec 06, 2023 at 04:23:25PM +0000, Catalin Marinas wrote:
> On Wed, Dec 06, 2023 at 11:38:09AM -0400, Jason Gunthorpe wrote:
> > On Wed, Dec 06, 2023 at 04:18:05PM +0100, Lorenzo Pieralisi wrote:
> > > On Wed, Dec 06, 2023 at 11:05:56AM -0400, Jason Gunthorpe wrote:
> > > > On Wed, Dec 06, 2023 at 02:49:02PM +0000, Catalin Marinas wrote:
> > > > > BTW, on those Mellanox devices that require different attributes within
> > > > > a BAR, do they have a problem with speculative reads causing
> > > > > side-effects? 
> > > > 
> > > > Yes. We definitely have had that problem in the past on older
> > > > devices. VFIO must map the BAR using pgprot_device/noncached() into
> > > > the VMM, no other choice is functionally OK.
> > > 
> > > Were those BARs tagged as prefetchable or non-prefetchable ? I assume the
> > > latter but please let me know if I am guessing wrong.
> > 
> > I don't know it was quite old HW. Probably.
> > 
> > Just because a BAR is not marked as prefetchable doesn't mean that the
> > device can't use NORMAL_NC on subsets of it.
> 
> What about the other way around - would we have a prefetchable BAR that
> has portions which are unprefetchable?

I would say possibly.

Prefetch is a dead concept in PCIe, it was obsoleted in PCI-X about 20
years ago. No PCIe system has ever done prefetch.

There is a strong incentive to mark BAR's as prefetchable because it
allows 64 bit addressing in configurations with bridges.

So.. I would expect people have done interesting things here.

Jason